A common liquid crystal display device has a structure wherein a pair of glass substrates having electrodes are faced each other, and the two glass substrates are bonded and fixed to each other on the periphery by a seal material leaving a liquid crystal injection opening, and a liquid crystal is sandwitched therebetween, and the liquid crystal injection opening is sealed by a sealant. In order to maintain a uniform distance between the two substrates, plastic beads, etc., having a uniform particle diameter are dispersed as spacers between the substrates.
As the displaying system of the liquid crystal display device, for example, TN (Twisted Nematic) mode, STN (Super Twisted Nematic) mode, GH (Guest Host) mode, ECB (Electrically Controlled Birefringence) mode, FLC (Ferroelectric Liquid Crystal) mode, and other systems are available.
As the driving system of the liquid crystal display device, simple-matrix driving system or active-matrix driving system is adopted. A liquid crystal display device of simple-matrix type has a structure wherein a first substrate having Y electrodes which have been patterned in a band shape in a horizontal (Y) direction and a second substrate having X electrodes which have been patterned in a band shape in a vertical (X) direction are positioned facing each other so that the Y electrodes and the X electrodes are substantially orthogonal to each other, and a liquid crystal is sandwitched between the two substrates. When color displaying, a color filter of R, G, and B is provided on one of the substrates.
A liquid crystal display device of active-matrix type has a structure wherein a liquid crystal is sandwitched between (a) an active-matrix substrate provided with an active element such as thin film transistors (TFTs) of a semiconductive layer of amorphous silicon (a-Si), pixel electrodes and signal electrodes connected to the TFTs, and gate electrodes and (b) a counter substrate having counter electrodes facing the active-matrix substrate. When color displaying, a color filter of R, G, and B is provided on the counter substrate. Note that, since details of such common conventional liquid crystal display devices are described in Liquid Crystal Device Handbook (published by The Nikkan Kogyo Shimbun Ltd.), Liquid crystal display technology (published by Sangyo Tosho Ltd.), and other publications, detailed explanations thereof are omitted here.
Meanwhile, as we shift into the information age, a demand for finer and larger screen in display devices used for, for example, a television in AV equipment and a monitor in OA equipment has not been higher, and in order to meet such a demand, development and practical application of larger screen has been under way in various display devices such as CRT display, liquid crystal display device (LCD), plasma display, EL display, and LED display.
Because an increase in weight, dimensions, and power consumption are expected as a result of increasing the screen size, lighter, thinner, and less power consuming display devices are also in demand. The liquid crystal display device, compared with other display devices, has such an advantage that the dimension of the depth (thickness) can be made significantly thinner and therefore is light weight, allowing the device to fit in a small space with ease, and the power consumption is low, thus meeting the above requirement. Further, since a full-color image can be obtained with ease, the liquid crystal display device is suitable for large screen display devices such as a large monitor and a wall display device, and expectations are high in realizing a larger screen more than for other display devices.
For example, Japanese Unexamined Utility Model No. 191029/1985 (Jitsukaisho 60-191029) and Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849) disclose a liquid crystal display device in which a large screen is realized in a structure wherein one of a pair of substrates constituting the liquid crystal display device is composed of a plurality of small substrates which are connected to one another. Also, Japanese Unexamined Patent publication No. 122769/1996 (Tokukaihei 8-122769) discloses a liquid crystal display device in which a large screen is realized by connecting a plurality of liquid crystal panels without noticeable seams.
However, in the conventional liquid crystal display devices such as above, because the spacers for maintaining a uniform distance between a pair of substrates are provided by being dispersed over the substrates by a dispersing method of dry type or wet type, it is technically difficult to maintain a constant density of the spacers with respect to the entire surface of the display area, and the density variance causes nonuniform cell gap d (spacing between a pair of substrates).
Also, because the spacers are merely dispersed over the substrates instead of bonded and fixed thereto, in the event where an external pressure is applied on the liquid crystal panel, the spacers are moved and this creates variance in spacer density, and as a result the cell gap d is changed.
Because the coloring of display by liquid crystal is closely related to the thickness of the liquid crystal layer, a change in cell gap d in a display area causes nonuniform displaying and the display quality is lowered. Particularly, as the screen size of the liquid crystal display device is increased in response to the recent demand for larger screen, the liquid crystal display device becomes more susceptible to the effect of bowing and oscillation caused by the dead weight of the glass substrates constituting the liquid crystal panel, namely, the liquid crystal display device becomes more susceptible to the adverse effect of external pressure, and nonuniformity in cell gap d is more likely to be generated. As a result, the display quality is further deteriorated.
As means to solve such problems, a method for forming the spacers by photolithography technique is disclosed, for example, in Japanese Unexamined Patent publication No. 173221/1986 (Tokukaisho 61-173221) and Japanese Unexamined Patent publication No. 223922/1990 (Tokukaihei 2-223922). In this method, specifically, after subjecting an alignment film to aligning process, photosensitive polyimide or photoresist is applied and exposed through a mask, and spacers of polyimide are fixably provided except on an effective pixel region.
However, in this method, photosensitive polyimide or photoresist (spacer material) is directly applied on an alignment film which has been subjected to aligning process by rubbing method. Therefore, it is required to carry out an etching process, etc., after exposure to remove unnecessary portions using a solvent.
The solvent used here swells a high polymer film (alignment film) and frees a part of molecular structure that has an aligning effect. Therefore, it is unavoidable that the liquid crystal aligning ability in a uniform direction, as given by rubbing, based on intermolecular effect is lowered. Namely, by the solvent, the aligned state of the alignment film material of the liquid crystal is contaminated and impaired severely, and the alignment of the liquid crystal injected into the liquid crystal panel becomes nonuniform.
Meanwhile, Japanese Unexamined Patent publication No. 175133/1994 (Tokukaihei 6-175133) discloses a method in which photosensitive polyimide or photoresist is applied onto an alignment film material which has not been subjected to aligning process, and spacers of polyimide are formed except on the effective pixel region by exposure and etching through a mask, and then the alignment film material exposed is subjected to aligning process by rubbing method, thereby preventing contamination and impairment of the aligned state caused by the solvent and the etching process. However, because the aligning process by rubbing is carried out after forming the spacers in the form of pillars, there is a case where alignment nonuniformity is generated, originating from each spacer pillar. This is caused by a slight difference in the rubbing state between the periphery of each spacer and the other portions in the rubbing process as induced by differences in various conditions such as the height and shape of the spacers formed on the substrate, the rubbing direction, the diameter, rotation speed, and rotation direction of a rubbing roll, the material, length, and implant density of rubbing cloth hair, and the transport speed of the substrate. For this reason, it had been required to limit the positioning of the spacer pillars so that the alignment nonuniformity originating from the spacers does not affect the display pixel area.